The present invention is directed to the processing of wafers, substrates or disks, such as silicon wafers, and more specifically to cluster tool systems, equipment and methods for processing wafers prior to device formation.
Wafers or substrates with exemplary characteristics must first be formed prior to the formation of circuit devices. In determining the quality of the semiconductor wafer, the flatness of the wafer is a critical parameter to customers since wafer flatness has a direct impact on the subsequent use and quality of semiconductor chips diced from the wafer. Hence, it is desirable to produce wafers having as near a planar surface as possible.
In a current practice, cylindrical boules of single-crystal silicon are formed, such as by Czochralski (CZ) growth process. The boules typically range from 100 to 300 millimeters in diameter. These boules are cut with an internal diameter (ID) saw or a wire saw into disc-shaped wafers approximately one millimeter (mm) thick. The wire saw reduces the kerf loss and permits many wafers to be cut simultaneously. However, the use of these saws results in undesirable waviness of the surfaces of the wafer. For example, the topography of the front surface of a wafer may vary by as much as 1-2 microns (xcexc) as a result of the natural distortions or warpage of the wafer as well as the variations in the thickness of the wafer across its surface. It is not unusual for the amplitude of the waves in each surface of a wafer to exceed fifteen (15) micrometers. The surfaces need to be made more planar (planarized) before they can be polished, coated or subjected to other processes.
Planarizing processes include lapping or grinding, followed by polishing steps. A lapping process, for example, may be performed to control thickness and remove bow and warp of the silicon wafer. The wafer is simultaneously lapped on both sides with an abrasive slurry in a lapping machine. The lapping process may involve one or more lapping steps with increasingly finer polishing grit. The lapping process, however, is slow and must be followed by careful cleaning and etching steps to relieve stresses before the wafer is polished. These additional steps cause the conventional method to be expensive and time-consuming. Also, the etching process employed after lapping is undesirable from an environmental standpoint, because the large amount of strong acids used must be disposed of in an acceptable way.
In another method, a grinding process replaces the lapping procedure. A first surface of the wafer is drawn or pushed against a hard flat holder while the second surface of the wafer is ground flat. Current grinding technology uses an abrasive wheel with a circular shaped diamond segment (bit) pattern as shown in FIG. 1A. This practice, however, causes the center of the wafer to be in constant contact with the grind segments (FIG. 1B). The constant contact in the center of the wafer is believed to create excess grinding at the wafer center relative to the wafer edge. Such a result tends to cause greater subsurface damage near the wafer center. For these and other reasons, the above techniques are undesirable.
Additional deficiencies in the current art, and improvements in the present invention, are described below and will be recognized by those skilled in the art.
The present invention provides exemplary methods, systems and apparatus that provide improved substrate characteristics after grinding operations by avoiding or reducing overgrind damage to the wafers. In one embodiment, an apparatus for grinding a substrate according to the present invention includes a first spindle having an eccentric-shaped abrasive matrix coupled thereto and a second spindle adapted to hold a substrate to be ground. The second spindle is offset from the first spindle.
In one aspect, the grinder further comprises a rotation device for rotating the first and second spindles. In one aspect, the first and second spindles are adapted to rotate such that the abrasive matrix passes through a center of the substrate only a portion of the time during which the first spindle completes a 360 degree rotation.
Preventing constant abrasive contact with the wafer center is accomplished a number of ways in alternative embodiments. In one aspect, the first spindle has a circular shaped surface to which an eccentric-shaped abrasive matrix is coupled. In another aspect, the first spindle has an elliptical shaped surface. The abrasive matrix is coupled near an edge of the elliptical shaped surface. In a particular embodiment, the abrasive matrix comprises a diamond bit pattern, although other abrasive matrix may be used within the scope of the present invention.
In one aspect of the present invention, grinding systems and apparatus further include a translation device coupled to the first spindle and adapted to translate the first spindle in a back and forth motion, or side-to-side motion. Similarly, in one aspect the translation device is coupled to the second spindle and is adapted to translate the second spindle in a back and forth motion, or side-to-side motion. In a particular embodiment, both spindles are translated to further ensure the wafer center is not over ground or over stressed.
In one aspect, the eccentric-shaped abrasive matrix is selected from an elliptical shape and an oval shape. In another aspect, the eccentric-shaped abrasive matrix further comprises a random abrasive matrix pattern.
In one embodiment of the present invention, a substrate grinding apparatus includes a first spindle having an abrasive matrix coupled to a first spindle surface, and a second spindle adapted to hold a substrate to be ground. The first and second spindles have first and second axii of rotation, respectively. The abrasive matrix has a non-circular pattern.
In one aspect, the first and second axii of rotation are generally parallel, and the first spindle is offset from the second spindle. In another aspect, the first and second spindles are adapted to rotate such that the abrasive matrix passes through a center of the wafer or substrate only a portion of the time during which the first spindle completes a 360 degree rotation. Again, the abrasive matrix may comprise a random pattern abrasive matrix.
The present invention further provides exemplary methods of grinding a substrate. In one embodiment, a grinding method includes providing a grinding apparatus as described herein. The spindles are positioned such that the abrasive matrix is in contact with a surface of the substrate. The method includes simultaneously rotating the first and second spindles so that the abrasive matrix contacts the substrate surface, with the abrasive matrix passing through a center of the substrate surface for only a portion of a time the spindles are rotating. In this manner, overgrinding and undue substrate stress can be reduced or avoided.
In one aspect, the grinding method further includes translating the first spindle in a back and forth motion, side-to-side or other motion while the abrasive matrix is in contact with the surface. In another aspect, the translating occurs simultaneous with the first and second spindle rotation. In still another aspect, the first spindle has a circular shaped surface to which an eccentric-shaped abrasive matrix is coupled.
In another method of grinding a substrate according to the present invention, a substrate grinder is provided. The substrate grinder has a first spindle with an abrasive matrix coupled thereto, and a second spindle adapted to hold a substrate to be ground. The first spindle has a first axis of rotation, and the second spindle has a second axis of rotation. The method includes positioning the spindles such that at least a portion of the abrasive matrix is in contact with a substrate surface to be ground, simultaneously rotating the first and second spindles so that the abrasive matrix contacts the substrate surface, and translating the first spindle so that the abrasive matrix passes through a center of the substrate surface for only a portion of a time the spindles are rotating. In this manner, the substrate center is not over ground or over stressed.
Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims and the accompanying drawings.